The ability to move the fingers relatively independently of one another enables human and non-human primates to manipulate diverse objects in the environment and to execute an immense variety of movements and gestures. Lesions of the motor cortex, such as those associated with stroke, give rise to debilitating and persistent deficits in the ability to move fingers individually even though the capacity to flex and extend the fingers together recovers. Accordingly, much scientific attention has been directed toward understanding how motor centers of the brain impart the ability to separately control the fingers. The function of the peripheral motor apparatus, however, has been largely ignored in the interpretation of findings related to the organization of the motor cortex. In particular, the actuators primarily responsible for moving the fingers are single-bellied muscles that give rise distally to multiple tendons that insert onto all the fingers. While the current view is that these multiendoned muscles consist of distinct functional compartments that govern the motion of different digits, the investigators have recently shown that motor unit force in these muscles is not concentrated on a single tendon but is broadly distributed across many tendons. This observation promotes a number of questions about the neural and muscular control of the fingers that are addressed in the specific aims of this proposal. First, what specific factors contribute to the dispersal of motor unit force across multiple tendons in muscles that control the fingers? Second, to what extent is it actually possible to move the fingers independently. And third, what strategies are employed by the nervous system when attempting to move a single finger given the apparent absence of independent actuators of the digits? These questions will be addressed in human subjects using a variety of electrophysiological and biomechanical techniques, including intraneural and intramuscular microstimulation, cross-correlation analysis of spike trains recorded from concurrently active motor units, kinematic analysis, and electromyographic recordings from multiple muscles during the execution of individuated finger movements. The results from these studies will add to our comprehension of how the spinal and muscular systems are coordinated in the elaboration of finger movements and will provide an important framework for understanding how the motor cortex specifies voluntary motion of the hand.